Walter Schottky Institut, Physik Department, and Center of Nanotechnology and Nanomaterials, Technische Universität München, Garching 85748, Germany.
Nano Lett. 2011 Sep 14;11(9):3848-54. doi: 10.1021/nl2019382. Epub 2011 Aug 15.
We identify a new noncatalytic growth regime for molecular beam epitaxially grown GaAs nanowires (NWs) that may provide a route toward axial heterostructures with discrete material boundaries and atomically sharp doping profiles. Upon increase of the As/Ga flux ratio, the growth mode of self-induced GaAs NWs on SiO(2)-masked Si(111) is found to exhibit a surprising discontinuous transition in morphology and aspect ratio. For effective As/Ga ratios <1, in situ reflection high-energy electron diffraction measurements reveal clear NW growth delay due to formation of liquid Ga droplets since the growth proceeds via the vapor-liquid-solid mechanism. In contrast, for effective As/Ga ratios >1 an immediate onset of NW growth is observed indicating a transition to droplet-free, facet-driven selective area growth with low vertical growth rates. Distinctly different microstructures, facet formation and either the presence or absence of Ga droplets at the apex of NWs, are further elucidated by transmission electron microscopy. The results show that the growth mode transition is caused by an abrupt change from As- to Ga-limited conditions at the (111)-oriented NW growth front, allowing precise tuning of the dominant growth mode.
我们发现了一种新的非催化生长状态，用于分子束外延生长的砷化镓纳米线（NWs），这可能为具有离散材料边界和原子级尖锐掺杂分布的轴向异质结构提供了一条途径。在增加砷/镓通量比时，我们发现 SiO2 掩模 Si(111) 上自诱导 GaAs NW 的生长模式在形态和纵横比上表现出惊人的不连续转变。对于有效 As/Ga 比<1，原位反射高能电子衍射测量显示由于形成液态 Ga 液滴，NW 生长会明显延迟，因为生长过程是通过汽-液-固机制进行的。相比之下，对于有效 As/Ga 比>1，观察到 NW 生长立即开始，表明向无液滴、晶面驱动的选择性区域生长转变，垂直生长速率较低。通过透射电子显微镜进一步阐明了截然不同的微观结构、晶面形成以及 NW 顶端是否存在 Ga 液滴。结果表明，生长模式转变是由于（111）取向 NW 生长前沿的 As 到 Ga 限制条件的突然变化引起的，从而可以精确调整主导生长模式。
